Niobium sintered body, production method therefor, and capacitor using the same

ABSTRACT

A niobium sintered body which is prepared in such a manner that a niobium powder is sintered at a temperature of 500° C. to 2000° C. and allowed to stand at a maximum sintering temperature for 60 minutes to 150 minutes in the course of sintering. The niobium sintered body of the present invention is characterized in that a product (CV) of a capacitance (C) per unit mass and a forming voltage (V) is 90,000 μF·V/g or more, and a value obtained by dividing a product of a mean particle diameter (D 50 ) of a primary particle of said niobium powder and a leakage current (LC) by said CV is 5×10 −4  μm·μA (μF·V) or less. And there can be provided a well-balanced capacitor with respect to a preferably low leakage current value regardless of the large capacitance, that is, a highly reliable capacitance.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on the provisions of 35 U.S.C. Article 111(a)with claiming the benefit of filing dates of U.S. provisionalapplication Ser. No. 60/233,438 filed on Sep. 18, 2000 under theprovisions of 35 U.S.C. 111(b), pursuant to 35 U.S.C. Article 119(e)(1).

TECHNICAL FIELD

The present invention relates to a niobium sintered body that canprovide a preferable leakage current value in spite of the largecapacitance, a production method therefor, and a capacitor using thesintered body.

BACKGROUND ART

Capacitors for use in electronic apparatus such as portable telephonesand personal computers are required to be small in size and large incapacitance. Of those capacitors, a tantalum capacitor is preferablyused, because the capacitance is large, not in proportion to the size,and the tantalum capacitor also has good characteristics. The tantalumcapacitor usually employs a sintered body of tantalum powder as ananode. In order to increase the capacitance of the tantalum capacitor,it is necessary to increase the mass of the sintered body.

The increase in mass of the sintered body inevitably enlarges the shapeof the capacitor, so that the requirement for a small-sized capacitor isnot satisfied. One approach to solve these problems is a capacitor usinga material which has a greater dielectric constant than tantalum. Onematerial which has such a greater dielectric constant is niobium.

Japanese Laid-Open Patent Application No. 55-157226 discloses a methodfor producing a sintered element for a capacitor. This method comprisesthe steps of subjecting a niobium powder ranging from an agglomerate tofine particles with a particle diameter of 2.0 μm or less to pressuremolding and sintering, finely pulverizing the molded sintered body,connecting a lead to the finely pulverized particles of the sinteredbody, and thereafter sintering the connected body again. However, theabove-mentioned application does not describe detailed characteristicsof the obtained capacitor.

U.S. Pat. No. 4,084,965 discloses a capacitor using a niobium powderwith a particle diameter of 5.1 μm obtained from a niobium ingot throughhydrogenation and pulverizing. However, the niobium sintered body has ahigh LC value, so that the serviceability of the niobium sintered bodyis regarded as poor.

The inventors of the present invention have already proposed to improvethe leakage current characteristics (hereinafter referred to as an LCvalue) of niobium by partially nitriding the niobium and the other likemanners (Japanese Laid-Open Patent Application No. 10-242004, U.S. Pat.No. 6,115,235). The LC value can be further decreased, for example, byincreasing the sintering temperature in the preparation of theabove-mentioned niobium sintered body. However, with the increase ofsintering temperature, a product of a capacitance per unit mass of theobtained sintered body and a forming voltage to form a dielectric on thesurface of the sintered body (hereinafter abbreviated as a CV value)becomes smaller. As a result, it is difficult to achieve the final goal,that is, to obtain a well-balanced niobium sintered body having a highCV value and a low LC value. When a capacitor is made from a niobiumsintered body which has been prepared only with an aim to obtain a highCV value, there is a problem that a capacitor having an exceptionallylarge LC value will be produced.

Therefore, an object of the present invention is providing a niobiumsintered body with a preferable leakage current value (LC value) inspite of the large capacitance, a production method therefor, and acapacitor using the above-mentioned sintered body.

DISCLOSURE OF THE INVENTION

The inventors of the present invention have intensively studied theabove-mentioned problems, and found an unprecedented sintering methodsuitable for niobium powder used for capacitors where the niobium powderis allowed to stand at a maximum sintering temperature for predeterminedtime, and then accomplished the present invention.

Namely, the present invention provides the following niobium sinteredbody, a production method therefore, and a capacitor using the sinteredbody.

[1] A niobium sintered body prepared by sintering a niobium powder,wherein a product (CV) of a capacitance (C: μF/g) per unit mass and aforming voltage (V: volt(V)) is 90,000 μF·V/g or more, and a valueobtained by dividing a product of a mean particle diameter (D₅₀: m) of aprimary particle of the niobium powder and a leakage current (LC: μA/g)by the CV value is 5×10⁻⁴ μm·A/(μF·V) or less.

[2] The niobium sintered body as described in the above-mentioned aspect[1], wherein the niobium powder is partially nitrided.

[3] The niobium sintered body as described in the above-mentioned aspect[2], wherein a nitrogen content in the niobium powder is within a rangeof 20 mass ppm to 200,000 mass ppm.

[4] The niobium sintered body as described in the above-mentioned aspect[3], wherein a nitrogen content in the niobium powder is within a rangeof 500 mass ppm to 7,000 mass ppm.

[5] A method for producing a niobium sintered body comprising the stepof sintering a niobium powder at high temperature, wherein the niobiumpowder is sintered at a temperature of 500° C. to 2,000° C. and allowedto stand at a maximum sintering temperature for 60 minutes to 150minutes.

[6] A method for producing a niobium sintered body as described in theabove-mentioned aspect [5], wherein the niobium powder is sintered at atemperature of 900° C. to 1500° C. and allowed to stand at a maximumsintering temperature for 80 minutes to 130 minutes.

[7] The method for producing the niobium sintered body as described inthe above-mentioned aspect [5] or [6], wherein the niobium powder isgranulated to have a primary particle with a mean particle diameter of 3μm or less.

[8] The method for producing the niobium sintered body as described inthe above-mentioned aspect [7], wherein the niobium powder is granulatedto have a primary particle with a mean particle diameter of 3 μm to 0.1μm.

[9] The method for producing the niobium sintered body as described inany one of the above-mentioned aspect [5] to [8], wherein the niobiumpowder is partially nitrided.

[10] The method for producing the niobium sintered body as described inthe above-mentioned aspect [9], wherein a nitrogen content in theniobium powder is within a range of 20 mass ppm to 200,000 mass ppm.

[11] The method for producing the niobium sintered body as described inthe above-mentioned aspect [10], wherein a nitrogen content in theniobium powder is within a range of 500 mass ppm to 7,000 mass ppm.

[12] A capacitor comprising an electrode comprising the niobium sinteredbody as described in any one of the above-mentioned aspect [1] to [4], adielectric provided on the surface of the sintered body, and a counterelectrode provided on said dielectric.

[13] The capacitor as described in the above-mentioned aspect [12],wherein the dielectric comprises niobium oxide formed by electrolyticoxidation.

[14] The capacitor as described in the above-mentioned aspect [12],wherein the counter electrode comprises at least one material selectedfrom the group consisting of an electrolytic solution, an organicsemiconductor, and an inorganic semiconductor.

[15] The capacitor as described in the above-mentioned aspect [14],wherein the counter electrode comprises the organic semiconductor, whichis selected from the group consisting of an organic semiconductorcomprising benzopyrroline tetramer and chloranil, an organicsemiconductor comprising as the main component tetrathiotetracene, anorganic semiconductor comprising as the main componenttetracyanoquinodimethane, and an organic semiconductor comprising as themain component an electroconducting polymer prepared by doping a polymerhaving at least two repeat units represented by general formula (1) or(2) with a dopant:

wherein R¹ to R⁴ which may be the same or different each independentlyrepresents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms,or an alkoxyl group having 1 to 6 carbon atoms; X represents an oxygenatom, a sulfur atom, or a nitrogen atom; and R⁵, which is present onlywhen X is a nitrogen atom, represents a hydrogen atom or an alkyl grouphaving 1 to 6 carbon atoms, and R¹ and R², and R³ and R⁴ may beindependently combined to form a ring.

[16] The capacitor as described in the above-mentioned aspect [15],wherein the organic semiconductor is at least one material selected fromthe group consisting of polypyrrole, polythiophene, and substitutedderivatives thereof.

EMBODIMENT OF THE INVENTION

One embodiment for obtaining a sintered body according to the presentinvention will be explained.

A niobium powder serving as a raw material for preparing a sintered bodymay have a primary particle with a mean particle diameter of 3 μm orless, preferably within a range of 3 μm to 0.1 μm. When the meanparticle diameter exceeds 3 μm, the niobium powder is not preferablebecause it is difficult to obtain a sintered body having high CVcharacteristics and low LC characteristics, that is one of the objectsof the present invention.

The mean particle diameter herein used is a value D₅₀ (a particlediameter when the mass percentage reaches 50% by cumulative distributionby mass), which is measured using a particle size distribution measuringapparatus (trademark “Microtrac”). The niobium powder having such a meanparticle diameter can be prepared, for example, by reducing potassiumfluoroniobate with a sodium, by subjecting a hydride of a niobium ingotto pulverizing and dehydrogenation, or by subjecting niobium oxide tocarbon reduction. When the niobium powder is prepared by subjecting ahydride of a niobium ingot to pulverizing and dehydrogenation, a niobiumpowder with a desired mean particle diameter can be obtained byadjusting the degree of hydrogenation in the niobium ingot and thepulverizing time by a pulverizer.

The niobium powder of the present invention may have the above-mentionedmean particle diameter, and preferably, partially nitrided. In thiscase, the nitrogen content of the nitrided powder is within a range of20 mass ppm to 200,000 mass ppm (Hereinafter, the term “mass ppm” willbe simply referred to as “ppm”.).

A sintered body is prepared from the above-mentioned niobium powder, anda dielectric is formed on the surface of the sintered body to bedescribed later. When the LC value is measured in an aqueous solution ofphosphoric acid, it is preferable that the nitrogen content is 500 ppmto 7,000 ppm, and more preferably 500 ppm to 4,000 ppm in order toprovide a niobium sintered body of which the leakage current value (LC)can be lowered in spite of the large capacitance. The nitrogen contentherein used is the amount resulting from chemical nitriding, not fromadsorption by the niobium powder and physical doping into the niobiumpowder.

The niobium powder can be nitrided by any of, or in combination ofliquid nitriding, ion nitriding, and gas nitriding. Of these methods,the gas nitriding of niobium powder under a nitrogen gas atmosphere ispreferred because the apparatus is simple and the operation is easy. Forexample, the gas nitriding can be achieved by allowing theaforementioned niobium powder to stand in an atmosphere of nitrogen gas.In this case, a niobium powder having a desired nitrogen content can beobtained by allowing the niobium powder to stand for 60 hours or less attemperatures of 2,000° C. or less. The processing time can be shortenedby increasing the processing temperature.

The nitrogen content in the niobium powder can be controlled byverifying the temperature and the time for nitriding in preliminaryexperiments after measuring the particle diameters of particles to besubjected to nitriding.

The above-mentioned niobium powder may be granulated to have an adequateshape before use, or the granulated powder may be mixed with anappropriate amount of a niobium powder not undergoing granulation. Anyconventional granulating methods can be used. For example, a niobiumpowder not undergoing granulation is allowed to stand at hightemperatures under vacuum to cause agglomeration and solidification, andthereafter the agglomerate is subjected to disintegration.Alternatively, after a specific binder is mixed with a niobium powdernot undergoing granulation, the resultant mixture is subjected todisintegration. In the latter case, the niobium powder and the bindermay be kneaded using a solvent when necessary, and in this case,disintegration is carried out after the kneaded mixture is dried. As thebinder, poly(vinyl alcohol) or acrylic resin is usually employed. Thesolvent may be selected from the group consisting of acetone, alcohols,esters such as butyl acetate, water, and the like.

The niobium powder thus granulated may have a mean particle diameter of300 μm or less, preferably 200 μm or less, and more preferably in therange of 200 μm to 1 μm.

A sintered body of niobium powder according to the present invention canbe produced by sintering the aforementioned niobium powder. Forinstance, the sintered body can be produced by subjecting the niobiumpowder to pressure molding to have a predetermined shape, and heatingthe pressure-molded article to temperatures of 500° C. to 2000° C.,preferably 900° C. to 1500° C. under the pressure of 1.33×10⁻⁴ to1.33×10² Pa (Pascal), in a condition that a niobium sintered body isallowed to stand at a maximum sintering temperature for 60 minutes to150 minutes, preferably for 80 minutes to 130 minutes.

Here, the maximum sintering temperature of niobium sintering body meansmaximum temperature in the range of the sintering temperature condition.The above-mentioned maximum sintering temperature may have a variationwithin a temperature control width of ±25° C. in the sinteringapparatus, and may be determined by spontaneously controlling thetemperature to have a wave form such as a square wave, pulse wave, ortriangular wave within a temperature width of ±50° C. with respect tothe preset maximum temperature.

The lower limit of the sintering temperature depends on the meanparticle diameter of the niobium powder. For example, the smaller themean particle diameter of the niobium powder, the lower the lower limitof the sintering temperature.

When the sintering temperature is changed with the mean particlediameter being the same, the sintered body produced at low sinteringtemperatures shows a high LC value although the CV value can increase.Such a sintered body cannot stand the practical use as a material foruse in a capacitor. In the present invention, however, even if thesintering temperature is low, the niobium sintered body may be allowedto stand at a maximum sintering temperature for 60 minutes to 150minutes, preferably for 80 minutes to 130 minutes, which is preferableto obtain niobium sintered body for use in a capacitor. As a result, theCV value can be increased to 90,000 μF·V/g or more and the LC value canbe decreased. In other words, a value of D₅₀·LC/CV, that is, a measureof the LC properties in light of the mean particle diameter of theniobium powder and the CV value, can be decreased to 5×10⁻⁴ μm·μA/(μF·V)or less.

In general, the LC/CV value represents an LC value per unit surfacearea. When the niobium powders constituting the respective sinteredbodies have different particle diameters, it is considered that the LCvalues of the sintered bodies are changed even though the surface areasof the niobium powders are the same. This is because the niobium powdersare different in surface shape. The above-mentioned D₅₀·LC/CV value is ameasure of the LC properties with the surface shape being taken intoconsideration. When a capacitor is produced using a sintered body havinga D₅₀·LC/CV value of greater than 5×10⁻⁴ μm·μA/(μF·V), there is thepossibility that the capacitor will show an exceptionally high LC value,which is not desired.

No niobium sintered body having such a high capacitance and a low LC asdesired can be obtained when the niobium powder is allowed to stand at amaximum sintering temperature for less than 60 minutes, or more than 150minutes.

The manufacture of a capacitor will now be described.

The capacitor of the present invention comprises said niobium sinteredbody as one electrode, a dielectric provided on a surface thereof, and acounter electrode provided on the dielectric.

A lead wire with an appropriate shape and length made of a metal with avalve action, such as niobium or tantalum, is integrally molded with theniobium powder in the course of the above-mentioned pressure molding ofniobium powder so that a part of the lead wire may be inserted into themolded article. Namely, assembling is carried out so that the lead wirecan serve as a leader from the sintered body.

It is preferable to employ a dielectric comprising niobium oxide as thedielectric of the capacitor according to the present invention. Theniobium oxide used in the present invention is a general term for “anoxide of niobium”. The number of oxygen atoms bonded to the niobium atomis not limited. For example, the oxides of niobium include Nb₂O₅, NbO₂,NbO_(X)(X is in a range of 1.0 to 2.5).

The dielectric comprising niobium oxide can be easily obtained, forexample, by subjecting the niobium sintered body serving as the oneelectrode to chemical treatment in an electrolyte. For the chemicaltreatment of the niobium electrode in an electrolyte, an aqueoussolution of a protic acid, such as a 0.1% aqueous solution of phosphoricacid or sulfuric acid is commonly used. When the niobium electrode issubjected to chemical treatment in the electrolyte to obtain adielectric comprising niobium oxide, an electrolytic capacitor can beprovided with the niobium electrode serving as an anode.

The counter electrode for use in the capacitor of the present inventionis not particularly limited. For example, at least one material(compound) selected from the group consisting of an electrolyteconventionally known in the aluminum electrolytic capacitor industry, anorganic semiconductor, and an inorganic semiconductor is preferablyemployed.

Specific examples of the electrolyte include a mixed solution ofdimethylformamide and ethylene glycol in which an electrolyte ofisobutyltripropylammonium borotetrafluoride is dissolved in an amount of5% by mass, and a mixed solution of propylene carbonate and ethyleneglycol in which an electrolyte of tetraethylammonium borotetrafluorideis dissolved in an amount of 7% by mass.

Specific examples of the organic semiconductor include an organicsemiconductor comprising benzopyrroline tetramer and chloranil, anorganic semiconductor comprising as the main componenttetrathiotetracene, an organic semiconductor comprising as the maincomponent tetracyanoquinodimethane, and an organic semiconductorcomprising as the main component an electroconducting polymer preparedby doping a polymer having at least two repeat units represented by thefollowing general formula (1) or (2) with a dopant:

wherein R¹ to R⁴ which may be the same or different each representsindependently a hydrogen atom, an alkyl group having 1 to 6 carbonatoms, or an alkoxyl group having 1 to 6 carbon atoms; X represents anoxygen atom, a sulfur atom, or a nitrogen atom; and R⁵, which is presentonly when X is a nitrogen atom, represents a hydrogen atom or an alkylgroup having 1 to 6 carbon atoms, and R¹ and R², and R³ and R⁴ may beindependently combined to form a ring. Any conventional dopant can beused as the above-mentioned dopant.

Examples of the polymer having a repeat unit represented by generalformula (1) or (2) in number of 2 or more are polyaniline,polyoxyphenylene, polyphenylene sulfide, polythiophene, polyfuran,polypyrrole, polymethylpyrrole, and substituted derivatives andcopolymers thereof. Of those polymers, polypyrrole, polythiophene, andthe substituted derivatives thereof such aspoly(3,4-ethylenedioxothiophene) are preferable.

Specific examples of the inorganic semiconductor include an inorganicsemiconductor comprising as the main component lead dioxide or manganesedioxide, and an inorganic semiconductor comprising tri-iron tetroxide.Those semiconductors may be used alone or in combination.

When the organic or inorganic semiconductor having an electricconductivity of 10⁻² S·cm⁻¹ to 10³ S·cm⁻¹ is used, the impedance of theobtained capacitor becomes smaller, so that the capacitance can befurther increased at high frequencies.

When the counter electrode is solid, an electroconducting layer may beformed thereon to improve the electrical contact with an external leadsuch as a lead frame.

The electroconducting layer may be formed by solidification of anelectroconducting paste, plating, metal deposition, or formation of aheat-resistant electroconducting resin film. As the electroconductingpaste, silver paste, copper paste, aluminum paste, carbon paste, nickelpaste, and the like are preferable. Those pastes may be used alone or incombination. When two or more electroconducting pastes are used, thepastes may be mixed together or laminated as separate layers. Once theelectroconducting paste is applied, the paste is allowed to stand in theatmosphere or is heated for solidification. With respect to the plating,nickel plating, copper plating, silver plating, aluminum plating, andthe like are used. For the metal deposition, aluminum, nickel, copper,silver, and the like can be employed.

To be more specific, a capacitor is constructed in such a manner that analuminum paste and a silver paste are successively overlaid on thesecond electrode, and thereafter sealing is carried out using a materialsuch as an epoxy resin. The capacitor may be provided with a niobium ortantalum lead that is integrally molded and sintered together with theniobium sintered body, or welded later.

The capacitor of the present invention with such a structure asmentioned above can be used for various applications when the capacitoris sheathed with a resin mold, resin case, metallic case, resin dipping,or laminated film.

When the counter electrode is liquid, the capacitor composed of theelectrodes and the dielectric is held in a can electrically connected tothe counter electrode. In this case, the electrode using the niobiumsintered body is designed to be externally led out via the niobium ortantalum lead, and be insulated from the can with an electricalinsulating rubber or the like.

When the capacitor is produced using the niobium sintered body preparedby the method of the present invention, the capacitor with a largecapacitance and good leakage current characteristics can be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be explained more specifically withreference to the examples and comparative examples, but is notparticularly limited to the following examples. In the followingexamples and comparative examples, a nitrogen content in a powder, acapacitance of the sintered body, a leakage current value (LC value) ofthe sintered body, and a capacitance of the capacitor processed into achip capacitor are measured by the method described below.

(1) Nitrogen Content in the Powder

The amount of nitrogen in the powder was measured using an apparatus fordetermination of oxygen and nitrogen based on thermal conductivity, madeby LECO corporation. The nitrogen content was expressed by the ratio ofthe amount of nitrogen to the weight of the powder separately measured.

(2) Capacitance of the Sintered Body

The sintered body was subjected to chemical treatment in a 0.1% aqueoussolution of phosphoric acid at 80° C. for 200 minutes to form adielectric on the surface of the sintered body. The above-mentionedsintered body immersed in a 30% aqueous solution of sulfuric acid and atantalum electrode in the sulfuric acid solution were connected with anLCR measuring apparatus “LCR meter” made by Hewlett Packard Co., Ltd.,to measure the capacitance at room temperature. The capacitance at 120Hz was regarded as a capacitance of the sintered body.

(3) Leakage Current Value (LC value) of the Sintered Body

The dielectric was formed as mentioned above(2). Then, a direct currentvoltage that was 70% of the forming voltage applied to form thedielectric was continuously applied for 3 minutes between theabove-mentioned sintered body immersed in a 20% aqueous solution ofphosphoric acid and an electrode in the phosphoric acid solution at roomtemperature. The current value measured three minutes later was regardedas a leakage current value of the sintered body.

(4) Capacitance of the Capacitor Processed into a Chip Capacitor

The capacitance of the capacitor processed into a chip capacitor was avalue measured at 120 Hz and at room temperatures using the LCR metermade by Hewlett Packard Co., Ltd. The leakage current value was acurrent value measured after the application of a rated voltage for 1minute.

In the following examples and reference examples, each of the CV and LCvalues was an average value measured from 20 capacitors.

EXAMPLES 1 TO 3 AND COMPARATIVE EXAMPLES 1 TO 4

A niobium powder with a mean particle diameter of 1 μm obtained bysubjecting potassium fluoroniobate to sodium reduction was allowed tostand at 1050° C under vacuum of 1.33×10⁻⁴ Pa for 20 minutes.Thereafter, the niobium powder was granulated by disintegration to havea mean particle diameter of 150 μm. The granulated powder was allowed tostand at 300° C. in a stream of nitrogen for 1.5 hours, therebyobtaining a partially nitrided niobium powder with a nitrogen content of1600 ppm by mass. The niobium powder thus obtained was molded togetherwith a niobium lead wire with a diameter of 0.3 mm φ, so that a 1.8mm×3.5 mm×4.5 mm molded article was produced in such a configurationthat a part of the lead wire with a length of 3.5 mm was in the moldedarticle and the other part with a length of 6 mm was outside. Aplurality of these articles (20×18 units) were sintered at a maximumsintering temperature of 1150° C. under the application of a pressure of1.33×10⁻⁴ Pa, with the sintering time being changed as shown in TABLE 1,thereby obtaining sintered bodies. Each sintered body was subjected tochemical treatment in a 0.1% aqueous solution of phosphoric acid at 80°C. under the application of a voltage of 20 V for 200 minutes to deposita dielectric of niobium oxide on the surface of the sintered body.

TABLE 1 shows the CV value, LC value, and D₅₀·LC/CV value of each of thesintered bodies. Further, a 1:1 mixture of a 30% aqueous solution oflead acetate and a 30% aqueous solution of ammonium persulfate wasbrought into contact with the dielectric 20 times at 40° C., whereby amixture of lead dioxide (in an amount of 97% by mass) and lead sulfatewas deposited as the counter electrode. Thereafter, a carbon paste and asilver paste were successively overlaid on the counter electrode,followed by sealing with an epoxy resin. Thus, a capacitor was produced.TABLE 3 shows the capacitance of each of the produced capacitors (with asize of 7.3 mm×4.3 mm×2.8 mm) and the LC value obtained by theapplication of a voltage of 6.3 V. Those values are average values takenfrom 20 capacitors in each Example, provided that the capacitors havingLC values of more than 100 μA are omitted. The number of such omittedcapacitors is also shown in TABLE 3.

EXAMPLES 4 TO 6 AND COMPARATIVE EXAMPLES 5 TO 8

A niobium powder with a mean particle diameter of 0.7 μm obtained from ahydride of a niobium ingot through pulverizing and dehydrogenation wasallowed to stand at 950° C. under vacuum of 1.33×10⁻⁴ Pa for 20 minutes.Thereafter, the niobium powder was granulated by disintegration to havea mean particle diameter of 120 μm. The granulated powder was allowed tostand at 300° C. in a stream of nitrogen for 1.5 hours, therebyobtaining a partially nitrided niobium powder with a nitrogen content of2000 ppm by mass. The nitrided niobium powder was molded in the samemanner as in Comparative Example 1, and sintered at a maximum sinteringtemperature of 1050° C. under the application of a pressure of 1.33×10⁻⁴Pa, with the sintering time being changed as shown in TABLE 1, therebyobtaining sintered bodies. Each sintered body was subjected to chemicaltreatment in the same manner as in Comparative Example 1, and the CVvalue, LC value, and D₅₀·LC/CV value were measured. TABLE 1 shows theabove-mentioned values. The procedure of bringing a 1:1 mixture of a 10%aqueous solution of ammonium persulfate and a 0.5% aqueous solution ofanthraquinone sulfonic acid into contact with the dielectric, andexposing the dielectric to a pyrrole gas was repeated at least 5 times,whereby a counter electrode of polypyrrole was deposited. Then, acapacitor was produced in the same manner as in Comparative Example 1.TABLE 3 shows various values of the capacitor.

COMPARATIVE EXAMPLES 9 TO 12

A sintered body and a capacitor were produced in the same manner as inComparative Example 1 except that the maximum sintering temperature waschanged as shown in TABLE 2. TABLE 2 and TABLE 4 show various values ofeach of the sintered bodies and the capacitors.

TABLE 1 Sintering CV LC D₅₀ · LC/CV Time (min) (μF · V/g) (μA/g) (μm ·μA/(μF · V)) Comparative 20 95000 60 6.3 × 10⁻⁴ Example 1 Comparative 4097000 50 5.1 × 10⁻⁴ Example 2 Example 1 80 97000 45 4.6 × 10⁻⁴ Example 2100 95000 30 3.1 × 10⁻⁴ Example 3 130 90000 30 3.3 × 10⁻⁴ Comparative200 82000 25 3.0 × 10⁻⁴ Example 3 Comparative 400 60000 20 3.3 × 10⁻⁴Example 4 Comparative 20 140000 135 6.7 × 10⁻⁴ Example 5 Comparative 40152000 110 5.1 × 10⁻⁴ Example 6 Example 4 80 150000 90 4.2 × 10⁻⁴Example 5 100 145000 80 3.8 × 10⁻⁴ Example 6 130 120000 75 4.4 × 10⁻⁴Comparative 200 89000 70 5.5 × 10⁻⁴ Example 7 Comparative 400 80000 655.7 × 10⁻⁴ Example 8

TABLE 2 Maximum D₅₀ · LC/CV Sintering LC (μm · μA/ Temp (° C.) CV (μF ·V/g) (μA/g) (μF · V)) Comparative 950 130000 290 2.2 × 10⁻³ Example 9Comparative 1050 110000 130 1.2 × 10⁻³ Example 10 Comparative 1250 7300025 3.4 × 10⁻⁴ Example 11 Comparative 1350 45000 10 2.2 × 10⁻⁴ Example 12Comparative 1150 95000 60 6.3 × 10⁻⁴ Example 1

TABLE 3 Capacitance The Number of Samples with (μF) LC (μA) LC of morethan 100 μA Comparative 420 35 1/20 Example 1 Comparative 430 32 1/20Example 2 Example 1 432 15 0/20 Example 2 426 10 0/20 Example 3 400 130/20 Comparative 355 11 0/20 Example 3 Comparative 260 9 0/20 Example 4Comparative 624 55 1/20 Example 5 Comparative 672 49 1/20 Example 6Example 4 668 25 0/20 Example 5 640 20 0/20 Example 6 530 18 0/20Comparative 382 18 0/20 Example 7 Comparative 347 15 0/20 Example 8

TABLE 4 Capacitance The Number of Samples with (μF) LC (μA) LC of morethan 100 μA Comparative 560 62 2/20 Example 9 Comparative 481 58 2/20Example 10 Comparative 309 10 0/20 Example 11 Comparative 188 8 0/20Example 12 Comparative 420 35 1/20 Example 1

When the results of Comparative Example 1 are compared with those ofComparative Example 9 to 12, it is found that as the sinteringtemperature increases, the CV value decreases although the LC valuebecomes better, which drastically lowers the capacitance of thecapacitor. As is apparent from the values concerning the capacitors inExamples 1 to 6, the well-balanced capacitors with respect to the LCvalue and the CV value, more specifically, the capacitors including nocapacitor with an LC value of more than 100 μA and having a capacitanceof 400 μF or more can be obtained when the sintering is carried out at amaximum sintering temperature for 80 minutes to 130 minutes, and using aniobium powder provided with a CV value of 90,000 μF·V/g or more, and aD₅₀·LC/CV value of 5×10⁻⁴ μm·μA/(μF·V) or less.

INDUSTRIAL APPLICABILITY

The niobium sintered body of the present invention is completed afterbeing sintered at a temperature of 500° C. to 2000° C. and allowed tostand at a maximum sintering temperature for 60 minutes to 150 minutesin the course of preparation of the sintered body and characterized inthat a product (CV) of a capacitance (C) per unit mass and a formingvoltage (V) is 90,000 μF·V/g or more, and a value obtained by dividing aproduct of a mean particle diameter (D₅₀) of a primary particle of saidniobium powder and a leakage current (LC) by said CV is 5×10⁻⁴μm·μA/(μF·V) or less.

The capacitor comprising niobium sintered body of the present inventioncan be a well-balanced capacitor with respect to the capacitance and theLC value, that is, a highly reliable capacitor.

What is claimed is:
 1. A niobium sintered body prepared by sintering aniobium powder, wherein a product (CV) of a capacitance (C: μF/g) perunit mass and a forming voltage (V: volt(V)) is 90,000 μF·V/g or more,and a value obtained by dividing a product of a mean particle diameter(D₅₀: μm) of a primary particle of the niobium powder and a leakagecurrent (LC: μA/g) by the CV value is 5×10⁻⁴ μm·μA/(μF·V) or less. 2.The niobium sintered body as claimed in claim 1, wherein the niobiumpowder is partially nitrided.
 3. The niobium sintered body as claimed inclaim 2, wherein a nitrogen content in the niobium powder is within arange of 20 mass ppm to 200,000 mass ppm.
 4. The niobium sintered bodyas claimed in claim 3, wherein a nitrogen content in the niobium powderis within a range of 500 mass ppm to 7,000 mass ppm.
 5. A capacitorcomprising an electrode comprising a niobium sintered body prepared bysintering a niobium powder, wherein a product (CV) of a capacitance (C:μF/g) per unit mass and a forming voltage (V: volt(V)) is 90,000 μF·V/gor more, and a value obtained by dividing a product of a mean particlediameter (D₅₀: μm) of a primary particle of the niobium powder and aleakage current LC: μA/g by the CV value is 5×10⁻⁴ μm·μA/μF·V) or less,a dielectric provided on a surface of the sintered body, and a counterelectrode provided on the dielectric.
 6. The capacitor as claimed inclaim 5, wherein the dielectric comprises niobium oxide formed byelectrolytic oxidation.
 7. The capacitor as claimed in claim 5, whereinthe counter electrode comprises at least one material selected from thegroup consisting of an electrolytic solution, an organic semiconductor,and an inorganic semiconductor.
 8. The capacitor as claimed in claim 7,wherein the counter electrode comprises the organic semiconductor, whichis selected from the group consisting of an organic semiconductorcomprising benzopyrroline tetramer and chloranil, an organicsemiconductor comprising as the main component tetrathiotetracene, anorganic semiconductor comprising as the main componenttetracyanoquinodimethane, and an organic semiconductor comprising as themain component an electroconducting polymer prepared by doping a polymerhaving at least two repeat units represented by general formula (1) or(2) with a dopant:

wherein R¹ to R⁴ which may be the same or different each independentlyrepresents a hydrogen atom, an alkyl group having 1 to 6 carbon atoms,or an alkoxyl group having 1 to 6 carbon atoms; X represents an oxygenatom, a sulfur atom, or a nitrogen atom; and R⁵, which is present onlywhen X is a nitrogen atom, represents a hydrogen atom or an alkyl grouphaving 1 to 6 carbon atoms, and R¹ and R², and R³ and R⁴ may beindependently combined to form a ring.
 9. The capacitor as claimed inclaim 8, wherein the organic semiconductor is at least one materialselected from the group consisting of polypyrrole, polythiophene, andsubstituted derivatives thereof.
 10. The niobium sintered body asclaimed in claim 1, wherein the niobium powder is granulated to have aprimary particle with a mean particle diameter of 3 μm or less.
 11. Theniobium sintered body as claimed in claim 1, wherein the niobium powderis granulated to have a primary particle with a mean particle diameterof 3 μm to 0.1 μm.